netcat

NC(1)                     BSD General Commands Manual                    NC(1)

NAME
     nc -- arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl] [-m minttl]
        [-O length] [-P proxy_username] [-p source_port] [-q seconds]
        [-s source] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout]
        [-X proxy_protocol] [-x proxy_address[:port]] [destination] [port]

DESCRIPTION
     The nc (or netcat) utility is used for just about anything under the sun
     involving TCP, UDP, or UNIX-domain sockets.  It can open TCP connections,
     send UDP packets, listen on arbitrary TCP and UDP ports, do port scan-
     ning, and deal with both IPv4 and IPv6.  Unlike telnet(1), nc scripts
     nicely, and separates error messages onto standard error instead of send-
     ing them to standard output, as telnet(1) does with some.

     Common uses include:

           o   simple TCP proxies
           o   shell-script based HTTP clients and servers
           o   network daemon testing
           o   a SOCKS or HTTP ProxyCommand for ssh(1)
           o   and much, much more

     The options are as follows:

     -4      Use IPv4 addresses only.

     -6      Use IPv6 addresses only.

     -b      Allow broadcast.

     -C      Send CRLF as line-ending.  Each line feed (LF) character from the
             input data is translated into CR+LF before being written to the
             socket.  Line feed characters that are already preceded with a
             carriage return (CR) are not translated.  Received data is not
             affected.

     -D      Enable debugging on the socket.

     -d      Do not attempt to read from stdin.

     -F      Pass the first connected socket using sendmsg(2) to stdout and
             exit.  This is useful in conjunction with -X to have nc perform
             connection setup with a proxy but then leave the rest of the con-
             nection to another program (e.g. ssh(1) using the ssh_config(5)
             ProxyUseFdpass option).  Cannot be used with -U.

     -h      Print out the nc help text and exit.

     -I length
             Specify the size of the TCP receive buffer.

     -i interval
             Sleep for interval seconds between lines of text sent and re-
             ceived.  Also causes a delay time between connections to multiple
             ports.

     -k      When a connection is completed, listen for another one.  Requires
             -l.  When used together with the -u option, the server socket is
             not connected and it can receive UDP datagrams from multiple
             hosts.

     -l      Listen for an incoming connection rather than initiating a con-
             nection to a remote host.  The destination and port to listen on
             can be specified either as non-optional arguments, or with op-
             tions -s and -p respectively.  Cannot be used together with -x or
             -z.  Additionally, any timeouts specified with the -w option are
             ignored.

     -M ttl  Set the TTL / hop limit of outgoing packets.

     -m minttl
             Ask the kernel to drop incoming packets whose TTL / hop limit is
             under minttl.

     -N      shutdown(2) the network socket after EOF on the input.  Some
             servers require this to finish their work.

     -n      Do not do any DNS or service lookups on any specified addresses,
             hostnames or ports.

     -O length
             Specify the size of the TCP send buffer.

     -P proxy_username
             Specifies a username to present to a proxy server that requires
             authentication.  If no username is specified then authentication
             will not be attempted.  Proxy authentication is only supported
             for HTTP CONNECT proxies at present.

     -p source_port
             Specify the source port nc should use, subject to privilege re-
             strictions and availability.

     -q seconds
             after EOF on stdin, wait the specified number of seconds and then
             quit. If seconds is negative, wait forever (default).  Specifying
             a non-negative seconds implies -N.

     -r      Choose source and/or destination ports randomly instead of se-
             quentially within a range or in the order that the system assigns
             them.

     -S      Enable the RFC 2385 TCP MD5 signature option.

     -s source
             Send packets from the interface with the source IP address.  For
             UNIX-domain datagram sockets, specifies the local temporary
             socket file to create and use so that datagrams can be received.
             Cannot be used together with -x.

     -T keyword
             Change the IPv4 TOS/IPv6 traffic class value.  keyword may be one
             of critical, inetcontrol, lowcost, lowdelay, netcontrol,
             throughput, reliability, or one of the DiffServ Code Points: ef,
             af11 ... af43, cs0 ... cs7; or a number in either hex or decimal.

     -t      Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL re-
             quests.  This makes it possible to use nc to script telnet ses-
             sions.

     -U      Use UNIX-domain sockets.  Cannot be used together with -F or -x.

     -u      Use UDP instead of TCP.  Cannot be used together with -x.  For
             UNIX-domain sockets, use a datagram socket instead of a stream
             socket.  If a UNIX-domain socket is used, a temporary receiving
             socket is created in /tmp unless the -s flag is given.

     -V rtable
             Set the routing table to be used.

     -v      Produce more verbose output.

     -W recvlimit
             Terminate after receiving recvlimit packets from the network.

     -w timeout
             Connections which cannot be established or are idle timeout after
             timeout seconds.  The -w flag has no effect on the -l option,
             i.e. nc will listen forever for a connection, with or without the
             -w flag.  The default is no timeout.

     -X proxy_protocol
             Use proxy_protocol when talking to the proxy server.  Supported
             protocols are 4 (SOCKS v.4), 5 (SOCKS v.5) and connect (HTTPS
             proxy).  If the protocol is not specified, SOCKS version 5 is
             used.

     -x proxy_address[:port]
             Connect to destination using a proxy at proxy_address and port.
             If port is not specified, the well-known port for the proxy pro-
             tocol is used (1080 for SOCKS, 3128 for HTTPS).  An IPv6 address
             can be specified unambiguously by enclosing proxy_address in
             square brackets.  A proxy cannot be used with any of the options
             -lsuU.

     -Z      DCCP mode.

     -z      Only scan for listening daemons, without sending any data to
             them.  Cannot be used together with -l.

     destination can be a numerical IP address or a symbolic hostname (unless
     the -n option is given).  In general, a destination must be specified,
     unless the -l option is given (in which case the local host is used).
     For UNIX-domain sockets, a destination is required and is the socket path
     to connect to (or listen on if the -l option is given).

     port can be specified as a numeric port number or as a service name.
     Port ranges may be specified as numeric port numbers of the form nn-mm.
     In general, a destination port must be specified, unless the -U option is
     given.

CLIENT/SERVER MODEL
     It is quite simple to build a very basic client/server model using nc.
     On one console, start nc listening on a specific port for a connection.
     For example:

           $ nc -l 1234

     nc is now listening on port 1234 for a connection.  On a second console
     (or a second machine), connect to the machine and port being listened on:

           $ nc 127.0.0.1 1234

     There should now be a connection between the ports.  Anything typed at
     the second console will be concatenated to the first, and vice-versa.
     After the connection has been set up, nc does not really care which side
     is being used as a 'server' and which side is being used as a 'client'.
     The connection may be terminated using an EOF ('^D').

     There is no -c or -e option in this netcat, but you still can execute a
     command after connection being established by redirecting file descrip-
     tors. Be cautious here because opening a port and let anyone connected
     execute arbitrary command on your site is DANGEROUS. If you really need
     to do this, here is an example:

     On 'server' side:

           $ rm -f /tmp/f; mkfifo /tmp/f
           $ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

     On 'client' side:

           $ nc host.example.com 1234
           $ (shell prompt from host.example.com)

     By doing this, you create a fifo at /tmp/f and make nc listen at port
     1234 of address 127.0.0.1 on 'server' side, when a 'client' establishes a
     connection successfully to that port, /bin/sh gets executed on 'server'
     side and the shell prompt is given to 'client' side.

     When connection is terminated, nc quits as well. Use -k if you want it
     keep listening, but if the command quits this option won't restart it or
     keep nc running. Also don't forget to remove the file descriptor once you
     don't need it anymore:

           $ rm -f /tmp/f

DATA TRANSFER
     The example in the previous section can be expanded to build a basic data
     transfer model.  Any information input into one end of the connection
     will be output to the other end, and input and output can be easily cap-
     tured in order to emulate file transfer.

     Start by using nc to listen on a specific port, with output captured into
     a file:

           $ nc -l 1234 > filename.out

     Using a second machine, connect to the listening nc process, feeding it
     the file which is to be transferred:

           $ nc -N host.example.com 1234 < filename.in

     After the file has been transferred, the connection will close automati-
     cally.

TALKING TO SERVERS
     It is sometimes useful to talk to servers "by hand" rather than through a
     user interface.  It can aid in troubleshooting, when it might be neces-
     sary to verify what data a server is sending in response to commands is-
     sued by the client.  For example, to retrieve the home page of a web
     site:

           $ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80

     Note that this also displays the headers sent by the web server.  They
     can be filtered, using a tool such as sed(1), if necessary.

     More complicated examples can be built up when the user knows the format
     of requests required by the server.  As another example, an email may be
     submitted to an SMTP server using:

           $ nc [-C] localhost 25 << EOF
           HELO host.example.com
           MAIL FROM:<user@host.example.com>
           RCPT TO:<user2@host.example.com>
           DATA
           Body of email.
           .
           QUIT
           EOF

PORT SCANNING
     It may be useful to know which ports are open and running services on a
     target machine.  The -z flag can be used to tell nc to report open ports,
     rather than initiate a connection. Usually it's useful to turn on verbose
     output to stderr by use this option in conjunction with -v option.

     For example:

           $ nc -zv host.example.com 20-30
           Connection to host.example.com 22 port [tcp/ssh] succeeded!
           Connection to host.example.com 25 port [tcp/smtp] succeeded!

     The port range was specified to limit the search to ports 20 - 30, and is
     scanned by increasing order (unless the -r flag is set).

     You can also specify a list of ports to scan, for example:

           $ nc -zv host.example.com http 20 22-23
           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!
           nc: connect to host.example.com 23 (tcp) failed: Connection refused

     The ports are scanned by the order you given (unless the -r flag is set).

     Alternatively, it might be useful to know which server software is run-
     ning, and which versions.  This information is often contained within the
     greeting banners.  In order to retrieve these, it is necessary to first
     make a connection, and then break the connection when the banner has been
     retrieved.  This can be accomplished by specifying a small timeout with
     the -w flag, or perhaps by issuing a "QUIT" command to the server:

           $ echo "QUIT" | nc host.example.com 20-30
           SSH-1.99-OpenSSH_3.6.1p2
           Protocol mismatch.
           220 host.example.com IMS SMTP Receiver Version 0.84 Ready

EXAMPLES
     Open a TCP connection to port 42 of host.example.com, using port 31337 as
     the source port, with a timeout of 5 seconds:

           $ nc -p 31337 -w 5 host.example.com 42

     Open a UDP connection to port 53 of host.example.com:

           $ nc -u host.example.com 53

     Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as
     the IP for the local end of the connection:

           $ nc -s 10.1.2.3 host.example.com 42

     Create and listen on a UNIX-domain stream socket:

           $ nc -lU /var/tmp/dsocket

     Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
     port 8080.  This example could also be used by ssh(1); see the
     ProxyCommand directive in ssh_config(5) for more information.

           $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42

     The same example again, this time enabling proxy authentication with
     username "ruser" if the proxy requires it:

           $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

SEE ALSO
     cat(1), ssh(1)

AUTHORS
     Original implementation by *Hobbit* <hobbit@avian.org>.
     Rewritten with IPv6 support by
     Eric Jackson <ericj@monkey.org>.
     Modified for Debian port by Aron Xu <aron@debian.org>.

CAVEATS
     UDP port scans using the -uz combination of flags will always report suc-
     cess irrespective of the target machine's state.  However, in conjunction
     with a traffic sniffer either on the target machine or an intermediary
     device, the -uz combination could be useful for communications diagnos-
     tics.  Note that the amount of UDP traffic generated may be limited ei-
     ther due to hardware resources and/or configuration settings.

BSD                            December 27, 2018                           BSD